Method of manufacturing via interconnnection of glass-ceramic wiring board

ABSTRACT

In a ceramic wiring board which comprises a copper via, breaks and defects of via interconnections resulting from enlargement of copper particles in the interior of the via during sintering, are prevented.  
     For this purpose, alumina of mean particle diameter from 1 μm to 4 μm is disposed in the interior of the via interconnection after sintering at an average interval of 7.4 μm or less.

[0001] This application is a Divisional application of application Ser.No. 09/501,683, filed Feb. 10, 2000.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a ceramic multilayer wiring board, inparticular a ceramic wiring board using copper as a via interconnection,to a ceramic multilayer wiring board having a suitable post-sinteringvia microstructure, and to a copper paste for obtaining thismicrostructure.

[0003] Ceramic wiring boards having a multilayer structure are used inelectrical devices where modular wiring substrates are required for highintegration and high-speed processing, due to the need for making fineinterconnections. Copper is the material of choice for theseinterconnections due to its low specific resistance.

[0004] As substrate used as a support for interconnections, an inorganicmaterial having glass as its principal component is used as the glasscan be sintered at the same time as the copper of the interconnections.A borosilicate glass suitable for substrates is described in detail inJapanese Patent Laid-Open No. Hei 8-333157. Fillers which may be addedare disclosed in Japanese Patent Laid-Open No. Hei 8-18232.

[0005] Here, the method of manufacturing the substrate will be brieflydescribed.

[0006] Generally, the inorganic material is supplied in the formsubstrate is manufactured by the well-known green sheet method. Thismethod consists of the following steps.

[0007] (1) Making a slurry of the powdered inorganic material using anorganic binder and a solvent.

[0008] (2) Forming this slurry into the shape of a sheet.

[0009] (3) Opening vias (through holes) in the sheet.

[0010] (4) Embedding an interconnection paste in the vias.

[0011] (5) Forming an interconnection or other pattern on the sheetsurface with the interconnection paste.

[0012] (6) Laminating these sheets with interconnection patternstogether under pressure.

[0013] (7) Heat treating the resulting laminate.

[0014] In the above-mentioned heat treatment process, the organic binderin the laminate and the organic substance in the interconnection pastedecompose and are thus eliminated. At the same time, the inorganicmaterial in the laminate which is in a powdered state of aggregation andthe conducting metal in the conducting paste are sintered and becomefiner.

[0015] However, if the organic binder remains in the sintered compact,it will be converted to graphite, and the quality of the substrate andwiring after sintering will deteriorate. For this reason, sufficientbinder removal time is generally allowed in the sintering step, followedby a sintering period which has the main purpose of increasing thefineness.

[0016] This classical type of heat treatment profile is disclosed inJapanese Patent Laid-Open No. Hei 8-18232, etc. The binder is removed inan atmosphere at about 800° C. for 15 hours, and the product is kept inan atmosphere at about 1000° C. for 2 hours for sintering. Water vaporetc. is usually added to the processing atmosphere during theabove-mentioned binder removal.

[0017] However, when copper is used for the metallic material of theconductor, although sintering of the copper takes place starting fromapproximately 600° C., sintering of the glass ceramics itself begins ata higher temperature. This difference of sintering start times maycauses serious problems in the substrate, particularly in the conductoror at the interface between the conductor and the ceramics, so in thecase of copper paste, an attempt is often made to adjust the sinteringstart temperature of the ceramics.

[0018] As an example, a copper paste mixed with alumina of particle size0.1 μm to 1 μm is disclosed by Japanese Patent Publication 2584911.Also, a copper paste comprising copper oxide and glass frit is disclosedby Japanese Patent Laid-Open No. Hei 8-279666.

SUMMARY OF THE INVENTION

[0019] In producing a multilayer wiring board using the above-mentionedgreen sheet method, in the case of an alumina and copper mixture, it isdifficult to disperse fine alumina of particle size less than 1 μm inthe copper paste. For this reason, it is difficult to obtain desiredpaste printing properties required for processes such as embeddinginterconnection paste in vias, or forming interconnections or otherpatterns.

[0020] Moreover, copper oxide tends to discharge copper ions in theglass, and may produce a fine copper colloid in the ceramics dependingon the firing conditions. This causes deterioration of the insulatingproperties of the ceramics, and decreased hardness.

[0021] On the other hand, as the microinterconnections are formed andvia diameters reach about 50 μm, a new problem may arise in addition tothe above-mentioned difference of sintering start temperature.Specifically, if copper particles grow very large during their growthwhen the substrate is fired, they will reach a size of the same order asthat of the via diameter. As a result, after sintering, vias will beformed of several enlarged copper particles, particle interfaces willbreak down due to the effect of subsequent heat cycles, and breaks willtend to occur in the via interconnections. Moreover, there is also thedisadvantage that via interconnections may fall out of the via holes ofthe ceramic substrate.

[0022] As an example of one way of dealing with this copper particlediameter problem after sintering, a copper paste mixed with aluminumacid which generates alumina of sub-micron size in the sinter isdisclosed in Japanese Patent Laid-Open No. Hei 8-17217. However, aswater vapor is generated simultaneously during the alumina formingreaction, more voids than needed were produced in the copperinterconnections.

[0023] This invention aims to overcome the disadvantages of the priorart by suppressing the size of copper particles in the via to 20 μm orless, thereby reducing breaks in interconnections due to fractures atinterfaces of copper particles which grow during sintering, and reducingthe risk of fractured vias separating from the ceramic substrate.

[0024] To achieve this objective, alumina having an average particlesize of 1 μm to 4 μm was distributed in sintered copper at an intervalof 7.4 μm or less in terms of the average distance between particlecenters.

[0025] The reason why the copper particles grow large during sinteringis that the copper particle boundaries migrate through the copper,fusing with the surface of the sinter body or with other copper particleboundaries, and this leads to a decrease of copper particle interfacesin the sintered copper.

[0026] By distributing alumina of average particle size 1 μm to 4 μm inthe sintered copper at the aforesaid interval, the copper boundaries canno longer migrate, the copper interfaces do not decrease even at thehigh temperature of the sintering step, and the copper particles remainin a fine state of division. As a result enlargement of copper particlesis prevented, and fractures of via interconnections do not occur.

[0027] The inventors experimentally verified that migration of particleboundaries in sintered copper was inhibited by alumina particles havingthe aforesaid size in restricted shapes such as vias. The details ofthese experiments will now be described.

[0028] The test substrate was an ordinary glass ceramic substrate havingvias of diameter 60 μm and comprising 10-40 layers, these layers beinglaminated so that the vias were vertically connected with each otherright through the substrate from one surface to the other. It should benoted that the number of layers in the substrate is not limited to theabove, and it may comprise only one layer.

[0029] Next, after sintering this substrate under sintering conditionsknown in the art, it was cut so that the center line of the via appearedon the surface. The cut surface was polished by the ordinary method, andthen etched so that the copper particle boundaries could be clearlyseen.

[0030] Next, for 500 or more vias observed in this cut surface, theshapes of the copper particle boundaries therein were read by acomputer, and these shapes were accurately traced so as to calculate thesurface area of the copper particles.

[0031] The reason why, in evaluating the state of the viainterconnections formed inside the vias, the surface area of the copperparticles was used as a parameter instead of the diameter which hasusually been used in the past, is as follows.

[0032] In general, when a particle grows in a shape like a via, theshape often has an aspect ratio largely different from 1, and if theshape is assumed to be a circle, it is difficult to evaluate the copperparticles precisely. The inventors also considered not only the averagesize of a particle, but also the maximum size. This is because inconsidering enlargement reactions of particles, when there are severalthousand vias in the substrate, it is difficult to assess the quality ofthe vias using only a simple average. However, in this experiment, thesurface area of each copper particle in the observed via cross-sectionwas measured and recorded.

[0033] It was found that when the diameter of the via is 60 μm, if thecross sectional area of a copper particle exceeds 2000 μm², the copperparticle interface will be formed from one side edge to the other sideedge of the via cross-section, and as all the particles having across-sectional area exceeding 3000 μm² cut across the viacross-section, there is a very strong possibility that particles havingthis cross-section will lead to fracture of the via.

[0034] Therefore, the via was filled with a paste of copper particles ofdiameter 0.5 μm to 6 μm with the objective of making the cross-sectionalarea of all the copper particles in the via, 2000 μm². The variation ofparticle diameter during the sintering step was observed.

[0035]FIG. 1 shows the change in the average value and maximum value ofcopper particle cross-sectional area in the sintering step. As a result,the average value of the copper particle cross-sectional area afterprocessing for 10 hours in an atmosphere at 850° C. at which pointbinder removal is complete was 50 μm²,and the maximum value within theobserved range was 1000 μm². When 1000° C. was reached, the crosssectional area of a copper particle was 270 μm², and the maximum valuewas 5000 μm². Also, at 1000° C., the average and maximum values of thecross sectional area of copper particles increased with the heatingtime. In an atmosphere at 950° C., the average value was 250 μm² and themaximum value was 5000 μm² when left for 2 hours.

[0036] Hence, after binder removal, a significant increase in thecross-sectional area of copper particles was found, and especially attemperatures higher than 950° C. The change of particle cross-sectionalarea in the process occurs mostly in this temperature region, butconsidering the behavior of the maximum particle cross-sectional area,it was clear that the particle size must be controlled from atemperature lower than 950° C.

[0037] Next, FIG. 2 shows the variation of the cross sectional area ofthe copper particles in the via in a paste wherein alumina is added tothe copper. The results apply to the case when the mean particlediameter of the added alumina is 1 μm, 2 μm and 4 μm, and the additionamount was varied from 2 vol % to 10.5 vol % relative to the inorganicsubstance in the via.

[0038] It is clear from these results that, for all alumina particlesizes from 1 μm to 4 μm, the cross-sectional area of copper particlescan be markedly decreased by adding alumina. However, the decrease doesnot much depend on the addition amount of alumina particles, the averagevalue of cross-sectional area lying in the range 100 μm²-200 μm² and themaximum value lying in the range 500 μm²-1500 μm² whatever the additionamount and particle size of alumina.

[0039]FIG. 3 shows the structure seen when observing the viacross-section. When alumina is added, as shown in FIG. 3A, alumina 4 ispresent at the boundaries of copper particles 31 or in regions where thecopper particles 31 overlap (multiple points). The copper particles aremuch finer than in the case shown in FIG. 3B, where alumina is notadded. Moreover, there are no longer any particles which span the wholevia from one side edge to the other side edge of the via cross-section,as shown in FIG. 3 B.

[0040] Thus, by adding the alumina 4 having a particle size which iseasily dispersed in copper paste, the alumina 4 inhibits particleboundary migration of the copper particles 31, and therefore preventsenlargement of the copper particles.

[0041] The reason why the copper particle boundary stops in the vicinityof the alumina is that the interface energy of the copper changes nearthe alumina, as shown in FIG. 4.

[0042] Consider the case where a particle boundary 42 migrates from aposition 1 to a position 3 in the copper 41, and there are aluminaparticles 43 at an intermediate position 2, as shown in FIG. 4A. Whenthe particle boundary 42 arrives at this position 2, the cross-sectionalarea of the boundary 42 decreases by an amount corresponding to thecross-sectional area Sb of the alumina 43, as shown in FIG. 4B.

[0043] At the same time, an interface Sa is formed between the copper 41and the alumina 43. Hence, if there is a large difference of interfaceenergies between copper particles and between copper particles andalumina particles, the interface energy of the particle boundary 42 willeither become very large or will vary at the position 2 (FIG. 4C).

[0044] The change ΔE of interface energy per alumina particle may beexpressed by the following equation.

ΔE=Sb·Ecc−Sa·Eca   (1)

[0045] where Sa=interfacial area between copper and alumina,

[0046] Sb =cross-sectional area of alumina,

[0047] Ecc=energy per unit cross-sectional area of copper-copperinterface,

[0048] Eca=energy per unit cross-sectional area of copper-aluminainterface

[0049] Substances which cause this change of interface energy are notlimited to alumina. This is because, if their interface energy isdifferent from the interface energy between copper and copper, theenergy at position 2 shows an extreme value whether the energydifference is positive or negative.

[0050] When the change AE of this energy is larger than the energy whichmoves the boundary surface 42, the boundary surface 42 cannot move pastthe alumina 43. It moves inside the sintered compact at the same speedas the alumina 43, but as the speed of movement of the alumina 43 in thesintered compact is itself small, the movement of the boundary surface42 is consequently impeded by the alumina 43.

[0051] It may therefore be said that a more correct understanding ofthis phenomenon could be achieved by considering the relation betweennumbers of alumina particles and interface surface area rather than byconsidering the relation between alumina concentration and copperparticle cross-sectional surface area or particle diameter.

[0052] The inventors measured the particle boundary length Lgcorresponding to the cross-sectional area A of a measured copperparticle, and calculated a copper particle boundary surface area Sgv inone via. Sgv was calculated by multiplying the particle boundary surfacearea Sv per unit volume of a crystalline substance, given by equation 2below, by the volume of a via which is the subject of this experiment.

Sv=(4/π)·Lg/A   (2)

[0053] Here, Lg is the particle boundary line length appearing in theobserved surface area, and A is the observed surface area.

[0054] Equation 2 is widely supported by researchers in the field, andis for example reported in “Ceramic Processing” by Mizutani, Ozaki,Kimura and Yamaguchi (Gihodo, 1985).

[0055]FIG. 5 shows the result of plotting the copper particle boundarysurface area Sgv in the aforesaid via against the number of aluminaparticles in the via computed from the alumina concentration. Theplotted data can be approximated using a straight line of slope 43 (1000μm²/1000). This means that a migration of a 43 μm² particle boundarysurface area can be prevented per alumina particle, and if this regionis assumed to be a circle, it is equivalent to a diameter range of 7.4μm.

[0056] From the above discussion, the minimum amount of alumina requiredto suppress the migration of a copper particle boundary, or in otherwords, the enlargement of a copper particle in the via, can also becomputed. Specifically, this value means that alumina particles aredistributed in the via at an interval of 7.4 μm.

[0057] It is known that in a particle disperse system, equation 3 shownbelow holds between the average value λ of the distance between thenearest particles and the particle number density Nv (for example, R. T.DeHoff, F, N.Rhines et al, “Measurement Morphology”, Rogakuo Uchida(1983), etc., translated by Makishima, Shinohara and Komori). From this,if λ=7.4 μm, Nv may be calculated to be 4.2×10⁻⁴ (/μm³).

λ=0.554/(N v ^(⅓))  (3)

[0058] As stated above, it is not additive concentration but the numberdensity of added particles and their interval which are the essentialpoints of this invention. The above measurements were made in order tounderstand the phenomenon, and require a great amount of effort. Inpractice, the volume of additive will probably be computed andmanufacturing carried out to obtain the desired interval and numberdensity, so the relation of the number density Nv of addition particlesand volume % of added particles such as alumina is given as equation 4.In this equation 4, it is assumed that the added particles arespherical. Moreover, if the required minimum number densities 4.2×10⁻⁴(/μm³) are given as examples for particles of diameter 1 μm, 2 μm, 4 μm,the corresponding volumes are respectively 0.022, 0.18, 1.4 volume %.

C=100 (π/6)·d ³ ·Nv   (4)

[0059] Here, C is the volume % in the via of an added particle, d is thediameter of the added particle, and Nv is the particle number density.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

[0061]FIG. 1 is a figure which describes the cross-sectional areavariation (average value and maximum value) of via copper particles inthe sintering step in this embodiment.

[0062]FIG. 2 is a figure which describes the cross-sectional areavariation (average value and maximum value) of via copper particlesafter sintering relative to an added alumina concentration.

[0063]FIG. 3 is a figure which shows the cross-sectional structure ofvia copper particles after sintering. FIG. 3A is the case when aluminais added, FIG. 3B is the case where alumina is not added.

[0064]FIG. 4 is a figure for describing the energy change when aparticle interface migrates in a copper sintered compact.

[0065]FIG. 5 is a figure showing change of copper particle boundarysurface area relative to alumina particle number in a via aftersintering.

[0066]FIG. 6 is a figure which describes the state in the via aftersintering according to this embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0067] Hereafter, the embodiments will be described referring to thefigures and tables.

[0068]FIG. 6 shows the method of manufacturing a multilayer substratehaving a via cross-sectional microstructure shown in FIG. 6. Themanufacturing process used the ordinary green sheet method. The greensheet itself was manufactured from a slip having the composition shownin the following Table 1 using the doctor blade method known in the art.TABLE 1 Composition Weight % glass powder 22.4 mullite 27.6 water 33.8isopropyl alcohol 8.0 dispersants 0.1 binders 8.1

[0069] The thickness of the green sheet described above was about 200μm. The composition of the glass is shown in Table 2. Both the glass andmullite had particle diameters of 3 μm. TABLE 2 Composition SiO₂ B₂O₃Al₂O₃ Na₂O Weight % 80 13 3 4

[0070] As a reference example, the glass was manufactured using acomposition disclosed in Japanese Patent Laid-Open No. Hei 8-333157, andthe green sheet was manufactured using a composition disclosed inJapanese Patent Laid-Open No. Hei 6-227855. This showed that a viahaving the characteristics of this embodiment could be formed even ifthe composition of the glass or green sheet was changed, and therefore,the composition of the glass and green sheet does not impose alimitation on the manufacturing method of this embodiment.

[0071] Next, a via 1 of diameter 60 μm was opened in the aforesaid greensheet using a hole opening tool (commonly referred to as a punch).

[0072] This via was then filled with paste. One example of the pasteused in this embodiment was obtained by adding 2.6 g of the aluminapowder 4 of 2.1 μm mean particle diameter to 100 g of a mull substancecomprising 92 vol % copper of 3 μm mean particle diameter in a vehiclecomprising ethyl cellulose and 2, 2, 4-trimethyl-1,3-pentadiolmonoisobutyrate in a ratio of 1:9. Next, 1.75 g of ethyl cellulose ofviscosity 300000 mPa/s was added so that the viscosity was, for example,about 400000 to 500000 mPa/s, the product was blended for about 60minutes using a tub paddle machine known in the art, and the resultingmixture was homogenized in an ordinary vibrating stirrer.

[0073] Next, the aforesaid paste was embedded in the via 1 of diameter60 μm according to the criteria for screen-stencil. After filling thevia with the paste, a predetermined intrasurface interconnection 2 wasprinted on this sheet using copper paste. This operation was repeated,and the resulting laminate of 25 layers was stuck together underpressure at 130° C.

[0074] In sintering the laminate, as an example, it was kept forapproximately 10 hours in an atmosphere at 850° C. while the temperaturewas increased at a rate of 100° C./hour 10 hours, left for 2 hours in anatmosphere at 1000° C., and cooled at an average rate of 200° C./hour.When the laminate was kept for approximately 10 hours in an atmosphereat 300° C. to 850° C. during the aforesaid temperature raising step,operations were performed in an atmosphere of water vapor comprisingnitrogen at 0.4 atmospheres in terms of partial pressure, and in anatmosphere of pure nitrogen at other times.

[0075] After subjecting the substrate produced by the above-mentionedmethod to various tests, the substrate was cut and the state of the via1 was confirmed. In particular, it was verified that no fatalabnormalities such as disconnection of vital interconnections occurredin the laminated substrate even if a load of 3000 or more cycles wereapplied in a −50° C./150 heat cycle test.

[0076]FIG. 6 shows a schematic view of the via 1 in this embodiment. Itshould be noted the boundary lines of copper particles do not appearclearly if the substrate is merely cut and its cross-section polished,therefore it was immersed for several seconds in an etching fluidcomprising water, 28% ammonia, and 3% aqueous hydrogen peroxide in aratio of 50:50:1 by volume.

[0077] The via 1 was thus formed so that the interconnections 2 withinthe surface were connected between layers. The copper particles 31 inthe via were finer than the copper particles 32 in the surfaceinterconnections. Further, in the via 1, the alumina 4 was present atthe boundaries of the copper particles 31 or in regions where theparticle boundaries overlapped (multiple points), but were not presentinside the copper particles 31.

[0078] The cross-sections of more than about 100 of the vias 1 wereobserved, and of the copper particles 31 in the vias 1, none were foundto have a cross-sectional area exceeding 1500 μm². No cracks wereobserved in the vias 3. When a chemical analysis was performed on thisvia 1, the proportions of the copper 31 and alumina 4 were respectively94.1 and 5.9 in terms of volume %.

[0079] Other compositions different from the above were manufactured aspastes filling the vias by the same manufacturing method as that of thisembodiment, and the same effect was obtained. Specifically, thecompositions of the pastes filling the vias were obtained as follows.The alumina powder 4 of 2.1 μm mean particle diameter was added,together with ethyl cellulose of viscosity 300000 mPa/s, to 100 g of amull substance comprising 92 vol % copper of 3 μm mean particle diameterin a vehicle comprising ethyl cellulose and 2, 2,4-trimethyl-1,3-pentadiol monoisobutyrate in a ratio of 1:9. In paste(a), the addition amount of ethyl cellulose was 0.55 g, in paste (b),the addition amount of ethyl cellulose was 4.8 g, and in paste (c), theaddition amount of ethyl cellulose was 3.21 g. In all of the above viafilling pastes (a), (b), (c), the state of the vias 1 after sinteringwas identical to that shown in FIG. 6 of this embodiment, and none ofthe copper particles 31 in the via had a cross-sectional surface areaexceeding 1500 μm².

[0080] It was moreover verified by chemical analysis that the vias 1which had been filled with one of the via filling pastes (a), (b) or (c)and sintered, respectively contained 2.0, 4.1, 10.5 of the alumina 4 interms of volume %.

[0081] As described above, if dispersible alumina particles of suitablesize are mixed with a paste of copper particles, and the result is usedto fill vias in a substrate and sintered, the sizes of the copperparticles in the vias do not grow as large as the via diameter. Even ifa load of 3000 or more −50° C. /150° C. heat cycles is applied, it doesnot cause breaks in interconnections due to fracture at copper particleinterfaces. Also, the problem of broken vias falling out of ceramicsubstrates is thereby largely resolved.

[0082] Consequently, it was possible to construct a ceramic wiring boardhaving a multilayer structure having high reliability in application toelectronic instruments requiring a high degree of integration andhigh-speed processing.

[0083] While we have shown and described several embodiments inaccordance with our invention, it should be understood that thedisclosed embodiments are susceptible of changes and modificationswithout departing from the scope of the invention. Therefore, we do notintend to be bound by the details shown and described herein, but intendto cover all such changes and modifications that fall within the ambitof the appended claims.

What is claimed is:
 1. A method for manufacturing a via interconnectionof a glass-ceramic wiring board, comprising the steps of: blendingcopper powder with a vehicle comprising a high-polymer organicsubstance; adding a metal oxide powder to said vehicle and blendingthem; adjusting the viscosity of said vehicle by adding saidhigh-polymer organic substance and filling them to a via; and sinteringsaid via and forming said via interconnection.
 2. A method formanufacturing a via interconnection of a glass-ceramic wiring board,comprising the steps of: blending a copper powder with a vehiclecomprising a cellulose derivative, adding a high-polymer organicsubstance to said vehicle and blending them; adjusting the viscosity ofsaid vehicle by adding said cellulose derivative and filling them to avia; and sintering said via and forming said via interconnection.
 3. Amethod for manufacturing a via interconnection of a glass-ceramic wiringboard, comprising the steps of: blending a copper powder to a vehiclecomprising a cellulose derivative; adding a metal oxide powder having amean particle diameter of from at least 1 μm to at most 4 μm to saidvehicle and blending them; adjusting the viscosity of said vehicle byadding said cellulose derivative and filling them to a via; andsintering said via at a temperature of at least 900° C. to at most 1060°C., and forming said via interconnection.
 4. A method for manufacturinga via interconnection of a glass-ceramic wiring board according to claim3 , wherein, in the step of adding said metal oxide powder and blendingthem, said metal oxide existing in said via after sintering is added tosaid vehicle such that the number of said metal oxide powders existingin said via is more than 4.2×10⁻⁴/μm³.
 5. A method for manufacturing avia interconnection of a glass-ceramic wiring board, comprising thesteps of: blending a copper powder to a vehicle comprising a cellulosederivative; adding a metal oxide powder having a mean particle diameterof from at least 1 μm to at most 4 μm to said vehicle and blending them;adjusting the viscosity of said vehicle by adding said cellulosederivative and filling them to a via; and sintering said via at atemperature of from at least 900° C. to at most 1060° C. and formingsaid via interconnection, wherein said metal oxide is dispersed in saidvia such that the mean interval of said metal oxide is at most 7.4 μm.